Printed circuit boards are used in a wide variety of electronic devices. The boards serve to support the electronic circuit components of the devices while “printed” filaments of conductive material (called traces) on the surface of the insulating board substrate, supply power to and interconnect the circuit components mounted on the board surface. In multilayer boards, some of the traces run between insulating layers in the interior of the board.
Printed traces or coatings perform the same function as wires but have several advantages over wires. For instance, since the traces are printed on the board, they are considerably less bulky than wire. Printed traces also eliminate the need for labor required to interconnect electronic components with wire, such as cutting the wire to appropriate lengths, stripping insulation off the wire, soldering individual wires to component leads, etc.
The electrical circuits on a substrate (e.g., printed circuit boards) are generally manufactured using etching/lithography technologies that involve time consuming processes (i.e., electroplating, masking, etching, etc.) and expensive tooling. For example U.S. Pat. No. 4,448,804 (by Amelio, W. J. et. al), U.S. Pat. No. 6,046,107 (by Lee, C. et. al), U.S. Pat. No. 6,951,604 (by Katayama, N. et. al) detail the electroplating of metals onto non-conductive surfaces and flexible boards. The solvents used in etching process are often corrosive and limit the choice of substrates for printing conductive patterns. Consequently, conventional etching processes are not economically viable for rapid prototyping and/or low volume/customized manufacturing of electric circuits. For these reasons, direct printing of conductive patterns has attracted tremendous attention in recent years.
The printing of electrically conductive inks has been known for quite some time (dating back to one of the early U.S. Pat. No. 3,043,784 by Remer, R. K. which discloses the formulation of carbon particles based conductive inks). Further improvements to the concept of generating electrically conductive coatings including graphite, metal flakes and metal coated glass spheres could be found in the U.S. Pat. No. 4,410,307 (by Collins, E. J. et. al. in the context of flash lamp array circuits); U.S. Pat. No. 5,098,771 (by Friend, S. O. on the use of carbon fibrils for conductive inks), U.S. Pat. No. 6,555,024 (by Ueda, T. et. al on formulation of pressure sensitive conductive ink formulations). Further, U.S. Pat. No. 5,286,415 by Buckley, M. S. et. al., report the formulation of aqueous, silver metal based conductive inks for thick film formation. However, all of the above mentioned patents suffer from the limitations of using conductive particles (i.e., carbon or graphite or metal particles) that provide only certain range of conductivities (typically low when compared to metal traces) with low printing resolutions and have prolonged curing times. Further, the above ink formulations are amenable to a few specific printing methods, materials and coating thicknesses and have limited applications and relevance in the context of modern printed electronics (e.g., PCBs, digital displays, RFID tags and photovoltaic cells) that demand faster and cost effective manufacturing processes.
WO 03/050824 by Johan, L. et. al discloses the formation of conductive traces using polyanions and intrinsically conductive polymers. However, organic conductors have poor stabilities compared to their inorganic counterparts such as metals and metal oxides, which exhibit wide range of conductivities and a variety of functional properties. Therefore, the metal or metal oxides based conductive coatings disclosed herein offer distinct chemical, physical, electrical and functional properties that cannot be obtained by organic conductive coatings.
Park, B. K. et. al report the direct writing of copper conductive patterns in Thin Solid Films, 515, 2007 (7706-7711) using inkjet printing methods, where copper particles were dispersed in the premixed solvent followed by ball milling for 12 hours and filtering through a 5 μm nylon mesh prior to printing. Finally, the printed coatings are sintered at high temperature under vacuum conditions. Although, the patterns generated are granular and have reasonable conductivities, and resolutions, the cumbersome processes involved makes it impractical for any commercial applications.
Physical Vapor Deposition (PVD), sputtering and Chemical Vapor Deposition (CVD) and related techniques are known to produce high quality conductive patterns, but high equipment, materials, and related costs, and low productivity make them difficult to be cost competitive against direct printing methods that are continuous. Therefore, there exists a need for printing a broad range of conductive elements and patterns with varying degree of viscosities, chemical compositions and functionalities using a simple printing methodology that is faster, continuous and cost effective. The present invention of self-patterning substrates eliminates the limits of several of the existing electronic feature printing methodologies by accommodating a wide range of coating techniques, viscosities and chemical composition by taking advantage of pre-coated single or multilayer complementary reactant coatings formulations that are electrically resistant or insulating and generate conductive patterns/coatings in response an external source of energy.
The present invention provides an alternate direct printing method for generating conductive traces on rigid and flexible substrates that are pre-coated either in part and/or in full with single and/or multiple layers of complementary chemical reactant formulations, which are electrically resistant. The transformation of resistant layers selectively into conductive traces on demand by an external energy source (such as localized source of heat, spark, microwave, pressure, light or laser or optical or electromagnetic or photochemical radiation) results in required conductive patterns with high resolution. The complementary reactant formulations may also be coated onto rigid or flexible substrates bearing conductive surfaces so that the conductive layers could be selectively turned into electrically resistant layers in order to generate conductive patterns in a subtractive fashion. These self-patterning substrates offer rapid and cost effective methods for printing electrical circuits for a variety of applications, such as Printed Circuit Boards (PCBs), antennas (RFID), flex/micro circuits, customized/disposable electronics, digital displays, photovoltaics, transistors, medical diagnostics and drug delivery devices.